You could create a "green" block chain by launching a secure cryptographic enclave that broadcasts a randomness beacon into orbit. (Several of them really, with the same seed.) Make it something that can be picked up by cheap SDRs. Then your block mining lottery becomes whoever can guess the next beacon output, and by being in orbit everyone can hear the beacon and it becomes extraordinarily costly (and obvious) to tamper with.<p>Of course that would allow extraterrestrials to cheat on coin mining...